Antibody Affinity Maturation in the Aging Bone Marrow

描述(申请人提供)：高亲和力抗原特异性抗体是保护性体液免疫的基础。这类抗体产生的缺陷是老年人对常见感染易感性增加的主要原因。抗体亲和力成熟是选择高亲和力免疫球蛋白(Ig)表达的B淋巴细胞的结果。大多数Ig基因在专门的淋巴生发中心经历抗原驱动的体细胞超突变(SHM)，以产生更高亲和力的抗体。我们在模型抗原为硝基苯基(NP)的小鼠身上的初步数据表明，在淋巴生发中心退化之后，抗体亲和力的成熟在初始反应的后期(免疫后35-70天)持续。令人惊讶的是，亲和力最高的抗体产生细胞主要在骨髓中被发现。由于相同的骨髓B细胞在典型的抗NP Ig可变区基因中也表现出较高的SHM发生率，我们推测SHM可能在骨髓中原位发生。SHM完全依赖于激活诱导的胞苷脱氨酶(AICDA)，该酶被认为几乎只在生发中心B细胞中表达。我们的分析表明，骨髓来源的、免疫球蛋白阳性的成熟B细胞也表达大量的AICDA，支持骨髓中存在原位Ig高突变的可能性。由于AICDA低表达和SHM差是老年人抗体亲和力成熟降低的原因，我们比较了青年(2月龄)和老年(22月龄)小鼠脾和骨髓B细胞中AICDA的表达水平。AICDA在老年脾组织中的表达显著降低，而骨髓中免疫球蛋白G阳性的B细胞在青年和老年之间的表达相似。基于这些发现，我们认为骨髓是抗体亲和力成熟的替代部位，在衰老过程中可能比其他外周淋巴部位保存得更好。在这个探索性的应用中，我们将通过比较年轻和老年骨髓中亲和力成熟的特征，通过识别迁移到骨髓的候选B细胞群体，并通过执行相互的、采用的转移来建立对骨髓的体液反应可能在衰老中受损的步骤(S)来检验这一假说。通过这些研究，我们希望开辟一个新的研究领域，我们可以在随后的更大范围内进行研究，以了解年轻人和老年人骨髓相关体液反应的细胞和分子机制，并测试新的疫苗接种策略，以提高老年人的抗体反应。

DESCRIPTION (provided by applicant): High affinity antigen-specific antibodies form the basis of protective humoral immunity. Defective generation of such antibodies is a major cause of the increased susceptibility of the elderly to common infections. Antibody affinity maturation is the result of the selection of high affinity immunoglobulin (Ig)-expressing B-lymphocytes. Most Ig genes undergo antigen-driven somatic hypermutation (SHM) in specialized lymphoid germinal centers to generate higher affinity antibodies. Our preliminary data in mice with the model antigen nitrophenyl (NP) suggest that antibody affinity maturation continues in the late phase of the primary response (35-70 days post immunization) well after the involution of lymphoid germinal centers. Surprisingly, the highest affinity antibody producing cells are found predominantly in the bone marrow. Because the same bone marrow B-cells also exhibited higher rate of SHM in the canonical anti-NP Ig variable genes, we hypothesized that SHM may take place in situ in the bone marrow. SHM is absolutely dependent on the activation-induced cytidine deaminase (AICDA) which is thought to be expressed almost exclusively in germinal center B-cells. Our analysis indicate that bone marrow-derived, IgG-positive, mature B-cells also express substantial level of AICDA, supporting the possibility of in situ Ig hypermutation in bone marrow. Because reduced antibody affinity maturation in the elderly is due to low level AICDA expression and poor SHM we compared the level of AICDA expression in splenic and bone marrow B-cells from young (2 month old) and old (22 month old) mice. While AICDA expression was significantly lower in the old spleen, it was comparable between young and old in bone marrow IgG-positive B-cells. Based on these findings we propose that the bone marrow is an alternative site of antibody affinity maturation which may, potentially, be better preserved during aging than the other, peripheral lymphoid sites. In this exploratory application we will test this hypothesis by comparing the characteristics of affinity maturation in young and old bone marrow, by identifying the candidate B-cell population that migrates to the bone marrow and by performing reciprocal, adoptive transfers to establish the step(s) where translocation of the humoral response to the bone marrow may be impaired in aging. With these studies we hope to open a new area of research which we could pursue in subsequent larger applications to understand the cellular and molecular mechanisms of bone marrow-associated humoral response in both young and aged individuals and test novel vaccination strategies that would improve antibody responses in the elderly.